In a wireless communication system, a transmitting terminal and a receiving terminal use multiple antennas to acquire high rates in a spatial multiplexing way. Compared with the general spatial multiplexing method, an enhanced technique is the receiving terminal feeding back channel information to the transmitting terminal, and the transmitting terminal uses a transmission precoding technique according to the acquired channel information, which can enhance the transmission performance greatly. With regard to Single-User Multi-input Multi-output (SU-MIMO), channel characteristic vector information is used directly to perform precoding; and with regard to Multi-User Multi-input Multi-output (MU-MIMO), comparatively accurate information is required.
In the 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), the feedback of channel information is mainly done by
a relatively simple feedback method of single codebook, and the performance of the transmission precoding technique of MIMO depends on the accuracy feedback from the codebook therein.
Here a basic principle of channel information quantization feedback based on the codebook is described briefly.
It is assumed that a limiting feedback channel capacity is B bps/Hz, the number of available code words is represented by N=2B. A characteristic vector space of channel matrix constitutes a codebook space ={F1F2, L, FN} through quantization. The transmitting terminal and the receiving terminal jointly save the codebook space  or produce the codebook space  in real time (the codebook space  is identical in the transmitting terminal and the receiving terminal). With regard to a channel H on which feedback of Channel State Information (CSI) is required to be performed, the receiving terminal selects a code word {circumflex over (F)} best matching with the channel H from the codebook space  according to a certain criterion, and feeds back a sequence number i of the code word {circumflex over (F)} (i.e. a sequence number of the code word) to the transmitting terminal. Here, the sequence number of code word is called as a Precoding Matrix Indicator (PMI) in the codebook. The transmitting terminal finds a corresponding precoding code word {circumflex over (F)} according to the sequence number i, thereby also acquiring the corresponding channel information. Wherein, {circumflex over (F)} indicates the characteristic vector information of the channel.
Generally, the codebook space  can further be divided into codebooks corresponding to multiple Ranks, and multiple code words corresponding to each Rank quantize precoding matrixes constituted by channel characteristic vectors under the Rank. Since the number of Ranks of the channel is equal to the number of nonzero characteristic vectors, when the number of Ranks is N, there will be N columns of the code words generally. Therefore, the codebook space  can be divided into multiple sub-codebooks according to the different Ranks, as shown in Table 1.
TABLE 1A schematic of dividing codebooks into multiple sub-codebooks accordingto Ranks Number of layers ν (Rank)12. . .N  1  2. . .  NCode word vectorCode word matrixCode word matrixset with column number 1set with columnset with columnnumber 2number N
Wherein, when Rank>1, the code words required to be stored are all in a matrix form, wherein this feedback method of codebook quantization is used for the codebooks in an LTE protocol. LTE downlink 4-transmitting antenna codebooks are as shown in Table 2, and the limitation of precoding codebook and channel information quantization codebook are identical in fact in LTE. Hereinafter, for the sake of uniformity, the vectors also can be regarded as a matrix whose dimensionality is 1.
TABLE 2A schematic of LTE downlink 4-transmitting antenna codebookCodebookTotal number of layers υindexun12340u0 = [1 −1 −1 −1]TW0{1}W0{14}/{square root over (2)}W0{124}/{square root over (3)}W0{1234}/21u1 = [1 −j 1 j]TW1{1}W1{12}/{square root over (2)}W1{123}/{square root over (3)}W1{1234}/22u2 = [1 1 −1 1]TW2{1}W2{12}/{square root over (2)}W2{123}/{square root over (3)}W2{3214}/23u3 = [1 j 1 −j]TW3{1}W3{12}/{square root over (2)}W3{123}/{square root over (3)}W3{3214}/24u4 = [1 (−1 − j)/{square root over (2)} −j (1 − j)/{square root over (2)}]TW4{1}W4{14}/{square root over (2)}W4{124}/{square root over (3)}W4{1234}/25u5 = [1 (1 − j)/{square root over (2)} j (−1 − j)/{square root over (2)}]TW5{1}W5{14}/{square root over (2)}W5{124}/{square root over (3)}W5{1234}/26u6 = [1 (1 + j)/{square root over (2)} −j (−1 + j)/{square root over (2)}]TW6{1}W6{13}/{square root over (2)}W6{134}/{square root over (3)}W6{1324}/27u7 = [1 (−1 + j)/{square root over (2)} j (1 + j)/{square root over (2)}]TW7{1}W7{13}/{square root over (2)}W7{134}/{square root over (3)}W7{1324}/28u8 = [1 −1 1 1]TW8{1}W8{12}/{square root over (2)}W8{124}/{square root over (3)}W8{1234}/29u9 = [1 −j −1 −j]TW9{1}W9{14}/{square root over (2)}W9{134}/{square root over (3)}W9{1234}/210u10 = [1 1 1 −1]TW10{1}W10{13}/{square root over (2)}W10{123}/{square root over (3)}W10{1324}/211u11 = [1 j −1 j]TW11{1}W11{13}/{square root over (2)}W11{134}/{square root over (3)}W11{1324}/212u12 = [1 −1 −1 1]TW12{1}W12{12}/{square root over (2)}W12{123}/{square root over (3)}W12{1234}/213u13 = [1 −1 1 −1]TW13{1}W13{13}/{square root over (2)}W13{123}/{square root over (3)}W13{1324}/214u14 = [1 1 −1 −1]TW14{1}W14{13}/{square root over (2)}W14{123}/{square root over (3)}W14{3214}/215u15 = [1 1 1 1]TW15{1}W15{12}/{square root over (2)}W15{123}/{square root over (3)}W15{1234}/2
Wherein, Wn=I−2ununH/unHun, I is a unit matrix, Wk(j) represents the jth column of vectors of the matrix Wk. Wk(j1j2. . . jn) represents a matrix constituted by the j1, j2, . . . , jn th column of the matrix Wk, and unH represents a conjugate transpose matrix of un; n represents a sequence number, and its values are 0˜15.
In an LTE standard, a minimum feedback object of the channel information is a Subband, the channel information can correspond to one Subband, or multiple Subbands, or all Subbands. One Subband consists of multiple physical Resource Blocks (RBs) defined by the LTE, and each RB consists of multiple Resource Elements (REs). An RE is the minimum unit (corresponding to the downlink resource allocation, wherein the minimum unit fed back in uplink is the Subband) of time-frequency resources in the LTE, and it continues to use a resource representation method of the LTE in the LTE-Advanced (LTE-A). All the Subbands under all the bandwidths are defined as Widebands, and multiple Subbands under part of the bandwidths are called as Multi-Subbands.
Certain contents related to the channel information feedback in the LTE will be introduced below, and the contents which are more concerned in the present document are Precoding Matrix Indicator (PMI) information of the channel.
The channel state information feedback includes: feedback of Channel Quality Indication (CQI), Precoding Matrix Indicator (PMI) and Rank Indicator (RI). Wherein,
The CQI is an index which measures the downlink channel quality. In the 36-213 protocol, the CQI is represented by integer values of 0˜15 which represent different CQI grades respectively, and different CQIs correspond to their respective Modulation and Coding Scheme (MCS).
The RI is used for describing the number of independent channels in the space and corresponds to a rank of a channel response matrix. A User Equipment (UE) is required to perform feedback of RI information in an open-loop spatial multiplexing mode and a closed-loop spatial multiplexing mode, and the RI information is not required to be fed back in other modes. The rank of channel matrix corresponds to a layer number.
The PMI information describes the characteristic vector information of the channel, and the UE reports the PMI information to a base station, which enables the base station to perform precoding better. The form of codebook feedback is used. This part of contents is also the most important contents of channel information feedback.
With the development of the communication technology, since the LTE-A has higher requirements on the spectrum efficiency, the number of antennas also increases to 8, and this requires designing 8-transmitting antenna codebook feedback to perform quantization feedback of the channel information. Moreover, in order to better support the MU-MIMO, the precision of the feedback also needs to be further enhanced. It is required to design a new codebook feedback scheme.
When it is 8-antenna, the application of dual-polarized antennas is more popularized, and single-polarized antennas also have certain application scenarios, thus it is required to design codebooks appropriate for dual-polarized channels and codebooks appropriate for single-polarized channels in the feedback technology.
In the related art, one idea is: changing the tradition that one 8-antenna codebook is constituted to perform feedback of the PMI channel information of the Subband, and instead, the UE feeding back one PMI1 corresponding to one 4-antenna codebook, and the base station finding a corresponding code word WPMI1 from the 4-antenna codebook; the UE also feeding back one PMI2, and the base station finding a corresponding code word WPMI2 from another 2-antenna codebook, and using the WPMI1 and WPMI2 to represent the channel information unitedly. The base station can combine the WPMI1 and WPMI2 according to an agreed function relationship and use the obtained matrix as the acquired channel information, what are obtained can be but not limited to one 8-antenna matrix, and the function relationship can be in a product form or a Kronecker product (whose operator is represented as ) form, and a principle thereof is as shown in FIG. 1.
4 antennas in the same polarization direction are equivalent to one virtual port (Virtual Port1), another 4 antennas in other polarization directions are equivalent to the other virtual port (Virtual Port2), which correspond to one 4-antenna codebook index PMI1 and one 2-antenna codebook index PMI2 respectively. According to the codebook indexes, the code word WPMI1 is found in the 4-antenna codebook and the code word WPMI2 is found in the 2-antenna codebook respectively, and thus, one 4-antenna codebook feedback and one 2-antenna codebook feedback replace one 8-antenna codebook feedback.
The characteristic vector information of the channel can be unitedly represented as:
            [                                                  WPMI              ⁢                                                          ⁢              1                                            O                                                O                                              WPMI              ⁢                                                          ⁢              1                                          ]        ⁢    WPMI    ⁢                  ⁢    2    ;
wherein, O represents a zero matrix, WPMI2 represents one 2×1 matrix (when it is Rank1) or one 2×2 matrix (when it is Rank2), that is, it corresponds to the code words in one 2-antenna codebook, and the UE performs feedback in the form of 2-antenna codebook.
The inventor of the application discovers that following problems exist in the related art.
Since the WPMI1 corresponds to the code words of one 4-antenna codebook, 8-antenna channel information cannot be quantized independently, and it is necessary to have information of the code words in the 2-antenna codebook corresponding to the WPMI2, and the multi-precision feedback cannot be provided. In addition, this method has a certain applicability with regard to the dual-polarized channels, but it cannot be applied to the single-polarized channels.
Moreover, the quantization accuracy of the code words in the existing codebooks is also required to be improved.